Heat exchangers play a vital part in ongoing efforts to conserve energy as well in as many thermodynamic systems. In any heat exchanger, the object is to achieve maximum heat transfer between heat exchange fluids with the minimum consumption of energy caused by flow resistance through the heat exchanger. It is also generally desirable to make the heat exchanger as compact as possible.
Recent developments in heat exchanger technology have predominantly concentrated in achieving higher heat transfer efficiencies by means of increased heat loads in more compact heat exchanger units. An effective approach to obtaining high levels of heat transfer in a compact volume is to maximize the contact area ratio between the hot fluid and the conducting material surface. This concept has led to the development of low profile plate-like heat exchangers consisting of large surface area conductive material inserts such as metallic foam or wire mesh, whose outer surfaces are sealed by means of brazed sheets. The highly irregular flow path within the heat exchanger increases mixing of the hot fluid and therefore promotes heat transfer to the conductive material insert and outer surfaces of the heat exchanger. Examples of such heat exchangers are shown in U.S. Pat. Nos. 6,305,079 and 5,983,992.
The effectiveness of such heat exchangers is greatly dependent on the nature and structure of the conductive material insert. In addition, various insert structures such as metallic foams, wire screens, and packed beds all feature different behaviors depending on the Reynolds number of the flow. For example, metal foam heat exchangers (MFHE) have been used extensively in cryogenics, power generation and many other fields requiring high heat load removal. They are relatively inexpensive, easy to form when mass-produced and capable of achieving surface area to volume ratios as high as 10,000 m2/m3. However, the inherent difficulty in manufacturing metal foam heat exchangers is how to maintain consistency in the geometry. Another limitation is that although models assume a well-aligned structure, actual MFHE cells are randomly positioned, which reduces the likelihood of direct conduction to the outer surfaces. Structures such as packed beds (sintered) outperform other heat exchanger designs in terms of a ratio of heat transfer to pressure loss. Although a packed bed of sintered metal balls may be more effective for heat transfer, its strength in tension (for use as a pressure vessel) is far less than wire screen.
The use of wire mesh as a conductive material insert in heat exchangers was first investigated in the late 1980s. Since then, many different mesh configurations have been considered and tested with varying degrees of success. Wire mesh heat exchangers (WMHE) have been shown to be more efficient than metal foam heat exchangers as they promote direct heat conduction to the outer walls while limiting axial conduction due to minimal contact between the screens. WMHEs are also easier to manufacture given that they are produced from readily available woven wire-screen textiles. These are relatively inexpensive and can easily be made of a wide variety of alloys. As manufacturing capabilities progress, more complex designs have emerged as attractive alternatives due to their potentially high Nusselt numbers and low friction factors. While these structures are expected to improve heat transfer efficiency, the potential slight gain in performance when compared to woven textiles may not justify the additional manufacturing costs.
Methods for producing WMHEs involve folding screens and brazing thin sheets to the tips of the folds. While these approaches are relatively simple to implement, they do however limit the conduction to the outer walls due to the smaller contact surface and the maximum number of times it can be folded per unit length. Another technique for making WMHEs considered by the inventor consists in sintering a stack of woven wire textiles, cutting them perpendicular to the stacked direction to form thin wafers, and then effectively sealing these wafers by brazing thin metal sheets on either sides. WMHEs manufactured by this method have been shown to successfully withstand internal pressures above 10000 psi. A drawback to this technique however is that the brazing process can be expensive due to high energy consumption, and this can mitigate against using WHMEs.